Solid oxide fuel cell and separator

ABSTRACT

A solid oxide fuel cell is formed by arranging a fuel electrode layer and an air electrode layer on both surfaces of a solid electrolyte, respectively, a fuel electrode current collector and an air electrode current collector outside the fuel electrode layer and the air electrode layer, respectively, and separators ( 8 ) outside the fuel electrode current collector and the air electrode current collector. In the first embodiment, a fuel gas and an oxidant gas are supplied from the separators ( 8 ) to the fuel electrode layers and the oxidant electrode layers, respectively, through the fuel electrode current collectors and the air electrode current collectors, respectively. Each separator ( 8 ) is formed by laminating a plurality of thin metal plates at least including a thin metal plate ( 21 ) in which a first gas discharge opening ( 25 ) is arranged in the central part and second gas discharge openings ( 24 ) are circularly arranged in the peripheral part, and a thin metal plate ( 22 ) with an indented surface. The weight saving of the electric power generation cell can be achieved, and the gases discharged from the separators ( 8 ) can be supplied to the whole areas of the electrode layers through the current collectors, so that an efficient electric power generation satisfactory in gas utilization ratio can be carried out. In the second embodiment, indents ( 8   a ) are provided on the surface of each of the separators ( 8 ), which surface is in contact with one of the current collectors ( 6 ), to increase the dwell volume and hence the retaining time of the gas in the interior of the current collectors. Thus, the gases permeate the interior of the current collectors slowly and are spread over the whole area of the current collectors, so that a satisfactory gas reaction can be carried out over the whole area of the electrode layers. Thus, the reaction time between the electrode layers and the gases can be made longer to thereby improve the electricity generation performance of the solid oxide fuel cell.

TECHNICAL FIELD

The present invention relates to a solid oxide fuel cell, morespecifically to a separator in a planar solid oxide fuel cell in whichthe introduced gas is supplied to the whole area of a current collectorto thereby equalize the imbalance in the electrode reaction, and theimprovement of the electric power generation efficiency is achieved.

BACKGROUND ART

The development of a solid oxide fuel cell, having a laminate structurein which a solid electrolyte layer made of an oxide ion conductor issandwiched between an air electrode layer (oxidant electrode layer) anda fuel electrode layer, is progressing as a third-generation fuel cellfor use in electric power generation. In a solid oxide fuel cell, oxygen(air) is supplied to the air electrode section and a fuel gas (H₂, COand the like) is supplied to the fuel electrode section. The airelectrode and the fuel electrode are both made to be porous so that thegases can reach the interfaces in contact with the solid electrolytelayer.

The oxygen supplied to the air electrode section passes through thepores in the air electrode layer and reaches the neighborhood of theinterface in contact with the solid electrolyte layer, and in thatportion, the oxygen receives electrons from the air electrode to beionized into oxide ions (O²⁻). The generated oxide ions move in thesolid electrolyte layer by diffusion toward the fuel electrode. Theoxide ions having reached the neighborhood of the interface in contactwith the fuel electrode react with the fuel gas in that portion toproduce reaction products (H₂O, CO₂ and the like), and release electronsto the fuel electrode.

The electrode reaction when hydrogen is used as fuel is as follows:

-   -   Air electrode: 1/2O₂+2e ⁻→O²⁻    -   Fuel electrode: H₂+O²⁻→H₂O+2e ⁻    -   Overall: H₂+½O₂→H₂O

Because the solid electrolyte layer is the medium for migration of theoxide ions and also functions as a partition wall for preventing thedirect contact of the fuel gas with air, the solid electrolyte layer hasa dense structure capable of blocking gas permeation. It is requiredthat the solid electrolyte layer has high oxide ion conductivity, and ischemically stable and strong against thermal shock under the conditionsinvolving the oxidative atmosphere in the air electrode section and thereductive atmosphere in the fuel electrode section; as a material whichcan meet such requirements, generally a stabilized zirconia (YSZ) thatis added with yttria is used.

On the other hand, the air electrode (cathode) layer and fuel electrode(anode) layer need to be formed of materials having high electronicconductivity. Because the air electrode material is required to bechemically stable in the oxidative atmosphere of high temperaturesaround 700° C., metals are unsuitable for the air electrode, andgenerally used are perovskite type oxide materials having electronicconductivity, specifically LaMnO₃ or LaCoO₃, or the solid solutions inwhich part of the La component in these materials is replaced with Sr,Ca and the like. Moreover, the fuel electrode material is generally ametal such as Ni or Co, or a cermet such as Ni—YSZ or Co—YSZ.

The solid oxide fuel cell is classified into the high temperatureoperation type operated at high temperatures around 1000° C. and the lowtemperature operation type operated at low temperatures around 700° C. Asolid oxide fuel cell of low temperature operation type uses an electricpower generation cell which is improved to work as a fuel cell even atlow temperatures by lowering the resistance of the electrolyte, forexample, through making the electrolyte made of an yttria stabilizedzirconia (YSZ) be a thin film of the order of 10 μm in thickness.

A solid oxide fuel cell operable at high temperature uses for theseparator, for example, a ceramic having electronic conductivity such aslanthanum chromite (LaCrO₃), while a solid oxide fuel cell of lowtemperature operation type can use for the separator a metallic materialsuch as stainless steel.

Additionally, as the structure of the solid oxide fuel cell, there havebeen proposed three types, namely, a cylindrical type, a monolithic typeand a flat plate type.

The stack of a solid oxide fuel cell has a structure in which electricpower generation cells, current collectors and separators arealternately laminated. A pair of separators sandwich an electric powergeneration cell from both sides of the cell in such a way one of theseparators is in contact with the air electrode through the intermediaryof an air electrode current collector while the other separator is incontact with the fuel electrode through the intermediary of a fuelelectrode current collector. For the fuel electrode current collector, aspongy porous substance made of a Ni based alloy or the like can beused, while also for the air electrode current collector, a spongyporous substance made of a Ag based alloy or the like can be used. Aspongy porous substance simultaneously displays current collectionfunction, gas permeation function, uniform gas diffusion function,cushion function, thermal expansion difference absorption function andthe like, and is accordingly suitable for a multifunction currentcollector.

The separators electrically connect between the electric powergeneration cells, and also have a function to supply the gas to theelectric power generation cells; therefore, each separator has a fuelpath through which the fuel gas is introduced from the peripheral sideof the separator and is discharged from the separator surface facing thefuel electrode layer, and an oxidant path through which the oxidant gasis introduced from the peripheral side of the separator and isdischarged from the separator surface facing the oxidant electrodelayer.

<Problems to be Solved by the Invention>

<First Problem>

In the case of the solid oxide fuel cell of low temperature operationtype, metal (stainless steel or the like) plates of the order of 5 to 10mm in thickness are used for the separators, and there has hitherto beenknown a separator having a structure such that gas discharge openings todischarge the fuel gas and the oxidant gas introduced from theperipheral side of the separator into the current collector are providedin the central part of the separator.

FIG. 8 is a sectional view of a relevant portion of a fuel cell stackillustrating an example of the above described separator. In FIG. 8,reference numeral 3 denotes a fuel electrode layer, reference numeral 6denotes a fuel electrode current collector, reference numeral 8 denotesa separator, reference numeral 11 denotes a fuel path, reference numeral25 denotes a gas discharge opening, and the arrows indicate the gaspermeation condition.

Here, it should be noted that such a conventional separator structure asdescribed above is associated with the following problems.

More specifically, the structure is such that the fuel gas dischargedfrom the central part of the separator 8 is supplied to the whole areaof the fuel electrode layer 3 through the fuel electrode currentcollector 6 made of a porous cushioning material; however, in practice,there is a problem in that the fuel gas is consumed to a large extent bythe electrode reaction in the neighborhood of the gas discharge opening25, and hence the gas concentration is decreased with increasingdistance away from the gas discharge opening 25. Consequently, theelectrode reaction is not uniformly conducted over the whole area of theelectrode, a temperature gradient is thereby generated in the electricpower generation cell, the electric power generation cell is sometimesbroken down by the thermal stress thus generated, and the resultinginefficient electric power generation leads to the degradation of theelectric power generation properties (the electricity production comesto be large in the central part of the electric power generation celland small in the peripheral part of the same cell). This problem hasbeen particularly conspicuous in the fuel electrode section.

Additionally, the use of thick metallic plates of 5 to 10 mm inthickness makes the weight of a single cell itself heavy, andaccordingly, in the case of a solid oxide fuel cell constructed bylongitudinally arranging cell stacks, there is a problem such that theelectric power generation cells in the cell stacks located in the bottomportion tend to be broken by the weight of the fuel cell. Consequently,as affairs stand, there remains a problem such that the cellconfiguration is inevitably constrained in such a way that the number oflamination is consistent with the tolerable weight of the fuel cell.Incidentally, in the case of a conventional structure, the weight of acell stack weighs about 1 kg, and the total weight of a cell module madeby laminating a large number of this cell stack comes to be about 25 kg.Consequently, the structure supporting such a module is naturallycomplex.

<Second Problem>

As described above, in a conventional solid oxide fuel cell, each of thecurrent collectors made of a porous cushioning material is arrangedbetween an electrode layer and a separator, and the gas is distributedto be supplied to each of the electrode layers through the currentcollectors; however, there has been a problem such that in theconventional structure, the retaining time of the gas in a currentcollector is short, and consequently the fuel gas not engaging with theelectrode reaction is discharged outside the electric power generationcell, so that the electric power generation efficiency is therebydegraded.

Additionally, in the conventional structure, the linear velocity of thegas in the peripheral part of the electric power generation cell comesto be slow; consequently there has also been a problem such that fromthe peripheral part of the electric power generation cell, air asoxidant is taken into the interior of the electric power generationcell, where the combustion reaction tends to take place, the combustionreaction completely consumes the fuel gas to be usable for the electrodereaction, and consequently the electric power generation efficiency isdegraded.

Such an adverse phenomenon has remarkably taken place particularly in afuel cell stack provided with the separators having a structure in whichthe fuel gas or the oxidant gas is supplied to the fuel cell electrodecurrent collector or the oxidant electrode current collector from thecentral part of each separator.

DISCLOSURE OF THE INVENTION

In view of the above described problems, a first object of the presentinvention is the provision of a planar solid oxide fuel cell in whichthe electric power generation efficiency is improved by uniformizing theelectrode reaction in the current collectors and adverse effects such asbreakdown accidents are prevented by making the separators light inweight, and the provision of the separator for use in the solid oxidefuel cell.

More specifically, the present invention according to claim 1 is aplanar solid oxide fuel cell in which a fuel electrode layer and anoxidant electrode layer are arranged on both surfaces of a solidelectrolyte layer, respectively; a fuel electrode current collector andan oxidant electrode current collector are arranged outside the fuelelectrode layer and the oxidant electrode layer, respectively;respective separators are arranged outside the fuel electrode currentcollector and the oxidant electrode current collector; and a fuel gasand an oxidant gas are supplied from the respective separators to thefuel electrode layer and the oxidant electrode layer respectively,through the fuel electrode current collector and the oxidant electrodecurrent collector, respectively, the fuel cell being characterized inthat each of the separators includes a first gas discharge opening fordischarging the introduced gas from the central part of the separatorand a plurality of second gas discharge openings for discharging theintroduced gas along the peripheral part of the separator in a circularmanner.

In the configuration described above, the gas is discharged from thecentral part of each separator and is discharged in a circular mannerfrom the peripheral part of each separator; accordingly, the gas can besufficiently supplied to and distributed over the whole areas of thecurrent collectors. Consequently, the electrode reactions are made to beperformed uniformly all over the whole areas of the electrodes; thus anefficient electric power generation can be carried out in which thedifference in electricity production between the central parts and theperipheral parts is eliminated.

Additionally, the present invention according to claim 2 ischaracterized in that in the planar solid oxide fuel cell according toclaim 1, the each separator is made up by laminating a plurality of thinmetal plates at least including a thin metal plate provided with thefirst gas discharge opening and the second gas discharge openings and athin metal plate with a worked indented surface.

According to the above described configuration, the separatorsthemselves can be made light in weight, the concavities and convexitiesof the thin metal plates form the gas flow paths and hence theintroduced gas is diffused uniformly over the whole areas of theseparators, so that ensured is the gas supply to the first gas dischargeopenings as a matter of course and also to the second gas dischargeopenings formed in the peripheral parts in a circular manner.

Additionally, the present invention according to claim 3 is a planarsolid oxide fuel cell according to claim 2, characterized in that thethin metal plate provided with the first gas discharge opening and thesecond gas discharge opening is arranged at least on the side of each ofthe fuel electrode current collectors.

The nonuniformity of the electrode reaction in the current collectors isconspicuous around the portions where the supplied gas enters. This isascribable to the fact that in contrast to air (the oxidant gas), thefuel gas cannot be supplied in large amount, so that the supply amountis restricted. Accordingly, in the present configuration, such gasdischarge structure as described above is applied at least to theseparator portions in contact with the fuel electrode currentcollectors, so that the nonuniformity of the electrode reaction in thefuel electrode layers is reduced.

Additionally, the present invention according to claim 4 is a separatorfor use in a solid oxide fuel cell which is contacted with each currentcollector arranged outside each electrode to form a gas passage forsupplying a gas to the electrode, characterized in that the separatorincludes a first gas discharge opening for discharging an introduced gasfrom the central part thereof and a plurality of second gas dischargeopenings for discharging the gas along the peripheral part thereof in acircular manner.

Additionally, the present invention according to claim 5 is theseparator for use in a solid oxide fuel cell according to claim 4,characterized in that the separator is made up by laminating a pluralityof thin metal plates including at least the thin metal plate providedwith the first gas discharge opening and the second gas dischargeopening and a thin metal plate having a worked indented surface.

Additionally, the present invention according to claim 6 is theseparator for use in a solid oxide fuel cell according to claim 5,characterized in that the thin metal plate provided with the first gasdischarge opening and the second gas discharge opening is arranged atleast on the side of the fuel electrode current collector.

Furthermore, in view of the above described problems involved in theconventional techniques, another object of the present invention is theprovision of a solid oxide fuel cell in which the electric powergeneration efficiency is improved by increasing the utilization ratiosof the fuel gas and the oxidant gas in the current collectors, and theprovision of the separator for use in the solid oxide fuel cell.

More specifically, the invention according to claim 7 is a solid oxidefuel cell in which a fuel electrode layer and an oxidant electrode layerare arranged on both surfaces of a solid electrolyte layer,respectively; a fuel electrode current collector and an oxidantelectrode current collector, both collectors being formed of a poroussubstance, are arranged outside the fuel electrode layer and the oxidantelectrode layer, respectively; respective separators are arrangedoutside the fuel electrode current collector and the oxidant electrodecurrent collector; and a fuel gas and an oxidant gas are supplied fromthe respective separators to the fuel electrode layer and the oxidantelectrode layer, respectively, respectively through the fuel electrodecurrent collector and the oxidant electrode current collector,respectively; the fuel cell being characterized in that indents areformed on the surface of each of the separators, which surface is incontact with each of the current collectors, to increase the dwellvolume of the gas in the current collectors.

In the above described configuration, the current collectors made of aspongy porous substance each are expanded in conformity with thedepression of the associated separator, and hence the volumes of theseparators are increased, so that the retaining time of the gas iselongated (the gas permeation rate is made slower) if the suppliedamount of the gas is constant. In this way, the reaction between thegases and the electrode layers comes to be conducted satisfactorily, andthe electric power generation efficiency is thereby improved.

Additionally, the invention according to claim 8 is a solid oxide fuelcell in which a fuel electrode layer and an oxidant electrode layer arearranged on both surfaces of a solid electrolyte layer, respectively; afuel electrode current collector and an oxidant electrode currentcollector, both collectors being formed of a porous substance, arearranged outside the fuel electrode layer and the oxidant electrodelayer, respectively; respective separators are arranged outside the fuelelectrode current collector and the oxidant electrode current collector;and a fuel gas and an oxidant gas are supplied from the respectiveseparators to the fuel electrode layer and the oxidant electrode layer,respectively through the fuel electrode current collector and theoxidant electrode current collector, respectively; the fuel cell beingcharacterized in that the peripheral part of the surface of each of theseparators, which surface is in contact with each of the currentcollectors, is protruded expandably to increase the linear velocities ofthe gases in the peripheral parts of the current collectors.

The increase of the linear velocity of the gas being discharged in theperipheral parts prevents the air entrained from the peripheral parts,and in particular, in the peripheral parts of the fuel electrode layers,can maintain the fuel gas concentration in an elevated concentrationcondition, and the electric power generation performance is therebyimproved.

Additionally, the invention according to claim 9 is a solid oxide fuelcell in which a fuel electrode layer and an oxidant electrode layer arearranged on both surfaces of a solid electrolyte layer, respectively; afuel electrode current collector and an oxidant electrode currentcollector, both collectors being formed of a porous substance, arearranged outside the fuel electrode layer and the oxidant electrodelayer, respectively; respective separators are arranged outside the fuelelectrode current collector and the oxidant electrode current collector;and a fuel gas and an oxidant gas are supplied from the respectiveseparators to the fuel electrode layer and the oxidant electrode layer,respectively, through the fuel electrode current collector and theoxidant electrode current collector, respectively; the fuel cell beingcharacterized in that indents are provided on the surface of each of theseparators, which surface is in contact with each of the currentcollectors, and the peripheral part of the separator is protrudedexpandably.

In the above described configuration, the gas permeation rate in theinterior of the current collectors is made slow and the electrodereactions are made satisfactory, and the linear velocity of the gas inthe peripheral parts is made fast, and the entraining of the air fromthe peripheral parts can thereby be prevented. Consequently, theelectric power generation performance can be improved.

Additionally, the present invention according to claim 10 is the solidoxide fuel cell according to any one of claims 7 to 9, characterized inthat the surface shape of the separators is formed at least on thesurfaces in contact with the current collectors.

The phenomenon of the incomplete reaction of the gas in the interior ofthe current collectors takes place on the portions where the suppliedfuel gas enters. This is ascribable to the fact that in contrast to air(the oxidant gas), the fuel gas cannot be supplied in large amount, sothat the supply amount is restricted. Accordingly, in the presentconfiguration, the depressions and the protruded portions are providedat least on the surface, in contact with one of the fuel electrodecurrent collectors, of each of the separator, and the phenomenon of theincomplete reaction of the gas and the phenomenon of the entraining ofthe air in the fuel electrode current collector are thereby remedied.

Additionally, the invention according to claim 11 is the solid oxidefuel cell according to any one of claims 7 to 10, characterized in thatthe fuel cell includes a structure in which the fuel gas and the oxidantgas are supplied from the central parts of the separators, respectively,to the fuel electrode layer and the oxidant electrode layer,respectively, through the fuel electrode current collector and theoxidant electrode current collector, respectively.

Additionally, the invention according to claim 12 is a separator for usein a solid oxide fuel cell which is in contact with one of the currentcollectors arranged outside the respective electrodes to form a gaspassage for supplying a gas to one of the electrode sections,characterized in that indents are provided on the surface of theseparator, which surface is in contact with one of the currentcollectors, to increase the dwell volume of the gas in the currentcollectors.

Additionally, the invention according to claim 13 is a separator for usein a solid oxide fuel cell which is contacted with each currentcollector arranged outside the each electrode to form a gas passage forsupplying a gas to each electrode section, characterized in that theperipheral part of the surface of the separator, which surface is incontact with the current collector, is protruded expandably to increasethe linear velocity of the gas in the peripheral part of the currentcollector.

Additionally, the invention according to claim 14 is a separator for usein a solid oxide fuel cell which is contacted with each currentcollector arranged outside each electrode to form a gas passage forsupplying a gas to each electrode section, characterized in that indentsare provided on the surface of the separator, which surface is incontact with the current collector, and the peripheral part of thesurface concerned is protruded expandably.

Additionally, the invention according to claim 15 is the separatoraccording to anyone of claims 12 to 14, characterized in that thesurface shape of the separator is formed at least on the surface incontact with one of the fuel electrode current collectors.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exploded oblique perspective view illustrating theconfiguration of a relevant portion of a planar solid oxide fuel cellinvolved in the present invention;

FIG. 2 a and FIG. 2 b illustrate the structure of a separator on theside of a fuel electrode involved in the present invention; FIG. 2 abeing a related plan view and FIG. 2 b being a related sectional view;

FIG. 3 is a sectional view of a relevant portion of a fuel cell stackinvolved in the present invention;

FIG. 4 is a sectional view of a relevant portion of a fuel cell stackillustrating the shape of the separator according to a second embodimentof the present invention;

FIG. 5 a to FIG. 5 d are sectional views illustrating the shapes ofseparators different from the shape shown in FIG. 1;

FIG. 6 is an exploded sectional view of a solid oxide fuel cell;

FIG. 7 is an exploded perspective view of a relevant portion of thesolid oxide fuel cell; and

FIG. 8 is a sectional view of a relevant portion of a conventional fuelcell stack.

BEST MODE FOR CARRYING OUT THE INVENTION

Description will be made below on the embodiments of the presentinvention with reference to the accompanying drawings. Incidentally, inthe following description, for the simplification of description, thesame reference symbols are used for the portions common to theconventional portions.

First Embodiment

Description will be made below on the first embodiment of the presentinvention with reference to FIG. 1, FIG. 2 to FIG. 2 b, and FIG. 3; inthe first place, on the basis of FIG. 1, description will be made on theconfiguration of a solid oxide fuel cell involved in the presentembodiment.

In FIG. 1, reference numeral 1 denotes a fuel cell stack, which has astructure in which an electric power generation cell 5 in which a fuelelectrode layer 3 and an air electrode layer (oxidant electrode layer) 4are arranged respectively on both surfaces of a solid electrolyte layer2, a fuel electrode current collector 6 arranged outside the fuelelectrode layer 3, an air electrode current collector (oxidant electrodecurrent collector) 7 arranged outside the air electrode layer 4, andseparators 8 arranged respectively outside the current collectors 6 and7 are laminated in this order. The present embodiment is suitablyapplicable to a sealless structure in which no gas seal is present alongthe rim of a fuel electrode current collector.

Here, the solid electrolyte layer 2 is formed of a stabilized zirconia(YSZ) that is added with yttria and the like, the fuel electrode layer 3is formed of a metal such as Ni or Co, or a cermet such as Ni—YSZ orCo—YSZ, the air electrode layer 4 is formed of LaMnO₃, LaCoO₃ or thelike, the fuel electrode current collector 6 is formed of a spongyporous sintered metal plate made of a Ni based alloy or the like, andthe air electrode current collector 7 is formed of a spongy poroussintered metal plate made of a Ag based alloy or the like.

The separators 8 have a function to connect electrically between theelectric power generation cells 5 similarly to the conventionalseparators, and also have a function to supply a gas to the electricpower generation cells 5; however, the structure of the separators isdifferent from the structure of the conventional separators shown inFIG. 8.

More specifically, the conventional separator is fabricated of a thick,single metal plate, whereas as shown in FIG. 2 a and FIG. 2 b, theseparator 8 of the present embodiment has a three layer structure whichis formed by successively laminating a metal upper plate 21 providedwith a plurality of gas discharge openings, an intermediate plate 22processed to have a surface with alternate convexities and concavities,and a flat lower plate 23. For all these plates, thin metal plates madeof stainless steel or the like are used.

In the upper plate 21, a first fuel gas discharge opening 25 is formedin the central part thereof, and a plurality of second fuel gasdischarge openings 24 are formed in a circularly aligned manner; thefuel gas introduced from the rim face of the separator 8 is discharged,through a fuel gas passage 11, from these gas discharge openings 24 and25, and supplied to the fuel electrode current collector 6 facing theseparator 8.

For the intermediate plate 22, three is used a sheet metal materialprocessed so as to have a surface with alternate convexities andconcavities for the purpose of ensuring the strength and the thicknessas a separator; this plate is combined with the upper plate 21 and thelower plate 23 to form a hollow separator 8 as shown in FIG. 2 b. Thehollow portions formed by these convexities and concavities function asthe gas flow path making the fuel gas diffuse easily, and simultaneouslythe weight saving of the separator 8 can be actualized.

Incidentally, the worked indented surface can be formed by applying theplastic working to this sheet metal; in contrast to the rectangularshape shown in the figure, an corrugated shape (corrugated plate) mayalso be used. Additionally, a plate material provided with workedindented surface patterns by applying the embossing processing may alsobe used.

The lower plate 23 forms a partition wall between the fuel electrodesection and the air electrode section. The above described combinationof the upper plate 21 and the intermediate plate 23 constitutes aseparator structure on the fuel electrode side; in practice, theseparator portion on the air electrode side is formed with theintervening lower plate 23, but in the figure concerned, the relevantportion is omitted.

Incidentally, the separators 8 (8A, 8B) at both ends of the fuel cellstack 1 shown in FIG. 1 have respectively either one of the abovedescribed separator structures on the fuel electrode side and the airelectrode side.

In the above described configuration of the planar solid oxide fuelcell, the fuel gas discharged from the central part and the peripheralpart of the separator 8 can be spread over the whole area of the fuelelectrode layer 3 with a satisfactory distribution through the fuelelectrode current collector 6; accordingly, the gas reaction can becarried out efficiently over the whole area of the electrode layer.

More specifically, a conventional type separator, provided with the gasdischarge opening 25 merely in the central part of the separator 8 shownin FIG. 8, has a structure such that the gas can be hardly spread to theperipheral part, and accordingly, the electrode reaction is notspatially uniform, so that there have been caused problems including thebreakdown of the electric power generation cell and the degradation ofthe electric power generation efficiency due to the thermal stress;however, according to the separator structure of the present embodiment,as shown in FIG. 3, the fuel gas introduced from the peripheral face ofthe separator through the fuel path 11 is made to diffuse over the wholearea of the separator by taking advantage of the hollow portions(convexities and concavities) of the separator 8 as the gas passage, thefuel gas is discharged from the first fuel gas discharge opening 25 inthe central part and the a plurality of second fuel gas dischargeopenings 24 in the peripheral part, and the fuel gas can be spread overthe whole area of the fuel electrode layer 3 with a satisfactorydistribution through the fuel electrode current collector 6 facing theseparator. Consequently, the electrode reaction comes to be carried outuniformly over the whole electrode areas, and hence the electric powergeneration can be carried out efficiently with a vanishing difference inelectricity production between the central part and the peripheral part.

Moreover, the separator 8 of the present embodiment is made to have alaminate structure with a hollow interior, and hence the weight of theseparator itself can be drastically reduced as compared to theconventional type separator. Such a structure is extremely effective ina fuel cell module having a structure in which a large number of cellstacks are longitudinally laminated, in view of the fact that the burdenloaded on the electric power generation cells located in the lowerpositions is reduced; consequently, the supporting frame for the fuelcell module can be simplified, and the constraint imposed on the numberof lamination in the cell stack can be alleviated. Thus, an electricpower generation of high electromotive force can be actualized.

As described above, as for the present embodiment, description has beenmade on the structure of the separator part in contact with the fuelelectrode current collector 6, and a similar structure can be applied tothe separator part in contact with the air electrode current collector7. Additionally, some simple discharge structure other than thosedescribed above (for example, as shown in FIG. 7, a gas dischargestructure restricted to the central part) can be adopted. Thenonuniformity of the electrode reaction in the interior of the currentcollectors is conspicuous around the portions where the supplied fuelgas enters, and accordingly, it is important to apply the structure ofthe present embodiment at least to the separator part facing the fuelelectrode current collector 6.

Additionally, in the present embodiment, the separator 8 has a threelayer structure formed of three thin metal plates; however, theseparator structure is not restricted to this structure, and may take atwo layer structure in which the lower plate 23 is omitted. In this way,a further weight saving of the separator 8 can be expected.

Additionally, in the present embodiment, there is presented a solidoxide fuel cell in which a stabilized zirconia. (YSZ) that is added withyttria is used for the electrolyte in the electric power generationcell; however, the present invention can be applied to other solid oxidefuel cells such as those solid oxide fuel cells in which a ceria basedelectrolyte and a gallate based electrolyte are used.

Second Embodiment

Now, description will be made below on the second embodiment of thepresent invention. FIG. 4 shows a sectional view of a relevant portionof a fuel cell stack illustrating the shape of the separator, FIG. 5 ato FIG. 5 d show the sectional views of the relevant portionsillustrating the other examples of separators, FIG. 6 shows an explodedsectional view of a solid oxide fuel cell, and FIG. 7 shows an explodedoblique perspective view of the relevant portion of the same solid oxidefuel cell in the present embodiment.

In the first place, on the basis of FIG. 6 and FIG. 7, description willbe made below on the configuration of the solid oxide fuel cell involvedin the present embodiment.

In FIG. 6, reference numeral 1 denotes a fuel cell stack, which has astructure in which an electric power generation cell 5 in which a fuelelectrode layer 3 and an air electrode layer (oxidant electrode layer) 4are arranged respectively on both surfaces of a solid electrolyte layer2, a fuel electrode current collector 6 arranged outside the fuelelectrode layer 3, an air electrode current collector (oxidant electrodecurrent collector) 7 arranged outside the air electrode layer 4, andseparators 8 arranged respectively outside the current collectors 6 and7 are laminated in this order.

The solid electrolyte layer 2 is formed of a stabilized zirconia (YSZ)that is added with yttria and the like, the fuel electrode layer 3 isformed of a metal such as Ni or Co, or a cermet such as Ni—YSZ orCo—YSZ, the air electrode layer 4 is formed of LaMnO₃, LaCoO₃ or thelike, the fuel electrode current collector 6 is formed of a spongyporous sintered metal plate made of a Ni based alloy or the like, theair electrode current collector 7 is formed of a spongy porous sinteredmetal plate made of a Ag based alloy or the like, and the separators 8are formed of a stainless steel or the like.

Here, the porous metal plates forming the current collectors 6 and 7 arethe plates having been fabricated through the following steps. The orderof the steps is as follows: a step for preparing a slurry→a step formolding→a step for foaming→a step for drying→a step for degreasing→astep for sintering.

In the first place, in the step for preparing a slurry, a metal powder,an organic solvent (n-hexane or the like), a surfactant (sodiumdodecylbenzenesulfonate or the like), a water soluble resin binder(hydroxypropylmethyl cellulose or the like), a plasticizer (glycerin orthe like) and water are mixed together, and thus a foaming slurry isprepared. In the step for molding, by means of the doctor blade method,the slurry is molded in a thin plate shape on a carrier sheet, and thusa green sheet is obtained. Then, in the step for foaming, this greensheet is foamed into a spongy condition in a high temperature and highhumidity environment with the aid of the vapor pressure of the volatileorganic solvent and the foaming property of the surfactant;subsequently, a porous metal plate is obtained through the step fordrying, the step for degreasing and the step for sintering.

In this case, in the step for foaming, the bubbles generated in theinterior of the green sheet grow with nearly spherical shapes as aresult of receiving nearly equivalent pressures along all thedirections. When a bubble diffuses to approach the interface to theatmosphere, the bubble grows toward the thin part of the slurryinterposed between the bubble and the atmosphere, and eventually thebubble is broken and the gas inside the bubble diffuses into theatmosphere through the formed small holes. Accordingly, there isobtained a porous metal plate provided with continuous pores havingopenings on the surface. The current collectors 6 and 7 each are formedby cutting a thus fabricated porous metal plate having a threedimensional skeleton structure into a circular form.

On the other hand, as shown in FIG. 6 and FIG. 7, the separators 8electrically connect between the electric power generation cells 5, andalso have a function to supply the gas to the electric power generationcells 5; therefore, each separator has a fuel path 11 through which thefuel gas is introduced from the peripheral side of the separator 8 andis discharged from the approximately central part of the surface of theseparator 8 facing the fuel electrode current collector 6, and anoxidant path 12 through which the oxidant gas is introduced from theperipheral side of the separator 8 and is discharged from the separatorsurface facing the air electrode current collector 7. Here, it should benoted that the separators 8 (8A, 8B) at both ends of the stack haverespectively either one of the paths 11 and 12.

Additionally, the separator 8 of the present embodiment is differentfrom a flat shaped conventional type shown in FIG. 8, the surface of theseparator 8 in contact with the fuel electrode current collector 6 ismade to be bowl shaped, as shown in FIG. 4, by providing a depression 8a with a deepened central part, and consequently the situation is suchthat the peripheral part 8 b is raised. As has already been described,the material for the fuel electrode current collector 6 itself is formedof a spongy foam, and hence, at the time of lamination, the foam isarranged in a condition such that the foam is in close contact with thedepression shape of the separator 8. Therefore, as far as the separator8 shown in FIG. 4 is used, the fuel electrode current collector 6 ismade to have a shape in which the central part of the collector isswollen as compared to a conventional collector (for example, if thethickness of a conventional fuel electrode current collector 6 is about0.75 mm, the maximum thickness of the central part is made to increaseto the order of about 1.5 mm in the case of the present embodiment), andmoreover, the peripheral part is made to be thinner as compared to theconventional type (for example, made to be of the order of 0.2 mm inrelation to the thickness of 0.75 mm of the conventional type).

Additionally, as shown in FIG. 6, respectively on both sides of the fuelcell stack 1, a manifold 15 for fuel for supplying fuel gas throughconnecting pipes 13 to fuel paths 11 in the respective separators 8 anda manifold 16 for oxidant for supplying oxidant gas through connectingpipes 14 to oxidant paths 12 in the respective separators 8 are arrangedalong the direction of the lamination of the electric power generationcells 5 in an extended manner.

According to the above described configuration of the fuel cell, thefuel gas discharged from the central part of the separators 8 is spreadover the whole area of the fuel electrode layer 3 through the fuelelectrode current collector 6 with a satisfactory distribution, and thusa satisfactory gas reaction can be carried out over the whole area ofthe electrode layer.

More specifically, as shown in FIG. 8, in a conventional type havingflat separators 8, the fuel electrode current collectors 6 are also flatshaped, and in particular, the permeation rate of the fuel gas (thearrows in the figure) is fast in the neighborhood of the central part ofeach of the fuel electrode current collectors 6 (in other words, theretaining time of the gas in the current collector is short); thus theelectrode reaction in the neighborhood of the central part of theelectrode layer is not completely carried out, and moreover, thesituation is such that the gas is not sufficiently spread to theperipheral part, so that the nonuniormity of the electrode reaction iscaused, and there is a possibility such that most of the fuel gas notengaged in the reaction is vainly discharged outside the electric powergeneration cell. On the contrary, the use of the separators 8 shown inFIG. 4 increases the volume of the fuel electrode current collectors 6themselves, so that if the supplied amount of the gas from theseparators 8 is constant, the permeation rate of the gas is thereby madeslower and the retaining time of the gas in the current collectors canbe made longer. Consequently, the gas discharged from the central partof each of the separators 8 can be made to permeate the wide area fromthe central part to the peripheral part of the fuel electrode currentcollector 6, and the fuel gas can thereby be supplied to the fuelelectrode layer 3 in a uniformly distributed manner, so that asatisfactory gas reaction can be carried out over the whole area of theelectrode layer.

Additionally, in each of the separators 8 of the present embodiment, theperipheral part is protruded expandably, and the thickness of theperipheral part of the fuel electrode current collector 6 thereby comesto be thinner than the thickness of the conventional type; therefore,particularly in the case of a sealless structure (a type in which therim of the fuel electrode current collector has no gas seal), the linearvelocity of the gas being discharged is increased in the peripheral partof the fuel electrode current collector, and the entraining of the airfrom the peripheral part is thereby prevented and the combustionreaction in the interior of the electric power generation cell can beinhibited, so that also in the peripheral part of the fuel electrodelayer 3, there can be maintained a condition in which the fuel gasconcentration is raised, and the improvement of the electric powergeneration performance can thereby be expected.

As described above, as for the present embodiment, description has beenmade on the shape of the surface, in contact with fuel electrode currentcollector 6, of the separator 8; the shape of the surface, in contactwith the air electrode current collector 7, of the separator 8 can bemade to have a similar shape. Additionally, the shape of the surface ofthe separator 8 is not limited to the shape shown in FIG. 4, and variousshapes as shown in FIG. 5 a to FIG. 5 d are conceivable. In thesefigures, reference numeral 8 a denotes a depression located in thecentral part or in the neighborhood thereof similarly to the casedescribed above, reference numeral 8 b denotes the peripheral partraised along the periphery of the depression 8 a. To sum up, acceptableis a shape in which the volume of the current collector can be madelarger, and thickness of the peripheral part can be made thin.

Additionally, as the porous structure of the current collectors 6 and 7,mesh, felt and the like can be used in addition to foam.

Additionally, in the present embodiment, there is presented a solidoxide fuel cell in which a stabilized zirconia (YSZ) that is added withyttria is used for the electrolyte in the electric power generationcell; however, the present invention can be applied to other solid oxidefuel cells such as those solid oxide fuel cells in which a ceria basedelectrolyte and a gallate based electrolyte are used.

INDUSTRIAL APPLICABILITY Effect of the First Embodiment

As described above, according to the present invention set forth inclaim 1 and claim 4, gas discharge openings are provided in the centralpart and the peripheral part of a separator, so that the gas can besufficiently spread over the whole area of a current collector.Consequently, the electrode reaction can be carried out uniformly overthe whole area of the electrode, and thus an efficient electric powergeneration can be carried out in which the difference in electricityproduction between the central part and the peripheral part of theelectric power generation cell is eliminated.

Additionally, according to the present invention set forth in claim 2and claim 5, the separators are made up by laminating a plurality ofthin metal plates including at least the thin metal plates each providedwith a first gas discharge opening and second gas discharge openings andthin metal plates having a worked indented surface; consequently theseparators themselves are made light in weight, and the number oflamination of a cell stack in a longitudinal type fuel cell module canthereby be increased, so that an electric power generation of highelectromotive force can be actualized. Additionally, the convexities andconcavities form the gas flow path, and hence the introduced gas comesto be easily supplied to the whole area of the current collector, sothat an efficient electric power generation can be actualized in whichthe nonuniformity of the electrode reaction in the interior of thecurrent collector is reduced.

Additionally, according to the present invention set forth in claim 3and claim 6, the above described separator structure according to claim1 and claim 2 is applied at least to the separator part on the side ofthe fuel electrode current collector, so that the nonuniformityphenomenon of the electrode reaction in the interior of the fuelelectrode current collector, which is conspicuous around the portionswhere the supplied gas enters, can be effectively improved, andconsequently an efficient electric power generation can be actualized inwhich the fuel utilization ratio is high.

Effect of the Second Embodiment

Additionally, according to the invention set forth in claim 7 and claim12, indents are provided on the surface, in contact with one of thecurrent collectors, of each of the separators, accordingly the dwellvolume of the gas in the interior of the current collectors isincreased, and hence the retaining time of the gas is thereby madelonger (the gas permeation rate is made slower). Consequently, the gasis slowly spread over a wide area through the current collector, asatisfactory gas reaction comes to be carried out over the whole area ofthe electrode layer. Accordingly, the fuel utilization ratio and the airutilization ratio are increased, and the electricity generationperformance is improved.

Additionally, according to the invention set forth in claim 8 and claim13, the peripheral part of the surface, in contact with the currentcollector, of the separator is protruded expandably, accordingly thelinear velocity of the gas being discharged is raised in the peripheralpart, the entraining of the air from the peripheral part is prevented,and the combustion reaction in the interior of the electric powergeneration cell can be inhibited. Consequently, in the peripheral partof the fuel electrode layer, there can be formed a condition in whichthe fuel gas concentration is raised, and the electric power generationperformance is thereby improved.

Additionally, according to the invention set forth in claim 9 and claim14, indents are provided on the surface, in contact with the currentcollector, of the separator, and the peripheral part of the separator isprotruded in an expanded manner; therefore, the effects set forth inclaim 1 and claim 2 are obtained in which the permeation rate of the gasin the interior of the current collector is made slower and theelectrode reaction is made satisfactory, moreover the linear velocity ofthe gas being discharged in the peripheral part is made fast, and theentraining of the air from the peripheral part can be prevented.

Additionally, according to the invention set forth in claim 10 and claim15, the above described surface shape of the separator is made to beformed at least on the surface thereof in contact with the fuelelectrode current collector, so that the phenomena of the incompletereaction of the gas and the entraining of the air in the fuel electrodecurrent collector are improved without failure, and hence the electricpower generation performance is improved.

Additionally, according to the invention set forth in claim 11, thestructure is such that the gases are supplied respectively from thecentral parts of the separators, respectively to the fuel electrodelayer and the oxidant electrode layer, respectively through the fuelelectrode current collector and the oxidant electrode current collector;therefore, the gases slowly permeate over the wide areas from thecentral parts of the current collectors to the peripheral parts, andsupplied to the electrode layers in a uniformly distributed manner, andsatisfactory electrode reactions come to be carried out over the wholeareas of the electrode layers.

1. A planar solid oxide fuel cell in which a fuel electrode layer and anoxidant electrode layer are arranged on both surfaces of a solidelectrolyte layer, respectively; a fuel electrode current collector andan oxidant electrode current collector are arranged outside said fuelelectrode layer and said oxidant electrode layer, respectively;respective separators are arranged outside said fuel electrode currentcollector and said oxidant electrode current collector; and a fuel gasand an oxidant gas are supplied from said respective separators to saidfuel electrode layer and said oxidant electrode layer, respectively,through said fuel electrode current collector and said oxidant electrodecurrent collector, respectively, wherein: each of said separatorscomprises a first gas discharge opening for discharging the introducedgas from the central part of said separator and a plurality of secondgas discharge openings for discharging the introduced gas along theperipheral part of said separator in a circular manner.
 2. The planarsolid oxide fuel cell according to claim 1, wherein said each separatoris made up by laminating a plurality of thin metal plates at leastincluding a thin metal plate provided with said first gas dischargeopening and said second gas discharge openings and a thin metal platehaving a worked indented surface.
 3. The planar solid oxide fuel cellaccording to claim 2, wherein the thin metal plate provided with saidfirst gas discharge opening and said second gas discharge openings isarranged at least on the side of each of said fuel electrode currentcollectors.
 4. A separator for use in a solid oxide fuel cell which iscontacted with each current collector arranged outside each electrode toform a gas passage for supplying a gas to the electrode, wherein: theseparator comprises a first gas discharge opening for discharging anintroduced gas from the central part thereof and a plurality of secondgas discharge openings for discharging the gas along the peripheral partthereof in a circular manner.
 5. The separator for use in a solid oxidefuel cell according to claim 4, wherein the separator is made up bylaminating a plurality of thin metal plates at least comprising a thinmetal plate provided with said first gas discharge opening and saidsecond gas discharge openings and a thin metal plate having a workedindented surface.
 6. The separator for use in a solid oxide fuel cellaccording to claim 5, wherein the thin metal plate provided with saidfirst gas discharge opening and said second gas discharge openings isarranged at least on the side of the fuel electrode current collector.7. A solid oxide fuel cell in which a fuel electrode layer and anoxidant electrode layer are arranged on both surfaces of a solidelectrolyte layer, respectively; a fuel electrode current collector andan oxidant electrode current collector, both collectors being formed ofa porous substance, are arranged outside said fuel electrode layer andsaid oxidant electrode layer, respectively; respective separators arearranged outside said fuel electrode current collector and said oxidantelectrode current collector; and a fuel gas and an oxidant gas aresupplied from said respective separators to said fuel electrode layerand said oxidant electrode layer, respectively, through said fuelelectrode current collector and said oxidant electrode currentcollector, respectively; wherein: indents are formed on the surface ofeach of said separators, which surface is in contact with each of saidcurrent collectors, to increase the dwell volume of the gas in saidcurrent collectors.
 8. A solid oxide fuel cell in which a fuel electrodelayer and an oxidant electrode layer are arranged on both surfaces of asolid electrolyte layer, respectively; a fuel electrode currentcollector and an oxidant electrode current collector, both collectorsbeing formed of a porous substance, are arranged outside said fuelelectrode layer and said oxidant electrode layer, respectively;respective separators are arranged outside said fuel electrode currentcollector and said oxidant electrode current collector; and a fuel gasand an oxidant gas are supplied from said respective separators to saidfuel electrode layer and said oxidant electrode layer, respectively,through said fuel electrode current collector and said oxidant electrodecurrent collector, respectively; wherein: the peripheral part of thesurface of each of said separators, which surface is in contact witheach of said current collectors, is protruded expandably to increase thelinear velocities of the gases in the peripheral parts of said currentcollectors.
 9. A solid oxide fuel cell in which a fuel electrode layerand an oxidant electrode layer are arranged on both surfaces of a solidelectrolyte layer, respectively; a fuel electrode current collector andan oxidant electrode current collector, both collectors being formed ofa porous substance, are arranged outside said fuel electrode layer andsaid oxidant electrode layer, respectively; respective separators arearranged outside said fuel electrode current collector and said oxidantelectrode current collector; and a fuel gas and an oxidant gas aresupplied from said respective separators to said fuel electrode layerand said oxidant electrode layer, respectively, through said fuelelectrode current collector and said oxidant electrode currentcollector, respectively; wherein: indents are provided on the surface ofeach of said separators, which surface is in contact with each of saidcurrent collectors, and the peripheral part of each of said separatorsis protruded expandably.
 10. The solid oxide fuel cell according toclaim 7, wherein the surface shape of said separators is formed at leaston the surface in contact with said current collectors.
 11. The solidoxide fuel cell according to claim 7, wherein the fuel cell comprises astructure in which said fuel gas and said oxidant gas are supplied fromthe central parts of said separators, respectively, to said fuelelectrode layer and said oxidant electrode layer, respectively, throughsaid fuel electrode current collector and said oxidant electrode currentcollector, respectively.
 12. A separator for use in a solid oxide fuelcell which is in contact with one of the current collectors arrangedoutside the respective electrodes to form a gas passage for supplying agas to one of the electrode sections, wherein: indents are provided onthe surface of said separator, which surface is in contact with one ofsaid current collectors, to increase the dwell volume of the gas in saidcurrent collectors.
 13. A separator for use in a solid oxide fuel cellwhich is contacted with each current collector arranged outside eachelectrode to form a gas passage for supplying a gas to each electrodesection, wherein: the peripheral part of the surface of the separator,which surface is in contact with said current collector, is protrudedexpandably to increase the linear velocity of the gas in the peripheralpart of said current collector.
 14. A separator for use in a solid oxidefuel cell which is contacted with each current collector arrangedoutside each electrode to form a gas passage for supplying a gas to eachelectrode section, wherein: indents are provided on the surface of theseparator, which surface is in contact with said current collectors, andthe peripheral part of the surface concerned is protruded expandably.15. The separator for use in a solid oxide fuel cell according to claim12, wherein said surface shape of the separator is formed at least onthe surface in contact with one of the fuel electrode currentcollectors.
 16. The solid oxide fuel cell according to claim 8, whereinthe surface shape of said separators is formed at least on the surfacein contact with said current collectors.
 17. The solid oxide fuel cellaccording to claim 9, wherein the surface shape of said separators isformed at least on the surface in contact with said current collectors.18. The solid oxide fuel cell according to claim 8, wherein the fuelcell comprises a structure in which said fuel gas and said oxidant gasare supplied from the central parts of said separators, respectively, tosaid fuel electrode layer and said oxidant electrode layer,respectively, through said fuel electrode current collector and saidoxidant electrode current collector, respectively.
 19. The solid oxidefuel cell according to claim 9, wherein the fuel cell comprises astructure in which said fuel gas and said oxidant gas are supplied fromthe central parts of said separators, respectively, to said fuelelectrode layer and said oxidant electrode layer, respectively, throughsaid fuel electrode current collector and said oxidant electrode currentcollector, respectively.
 20. The solid oxide fuel cell according toclaim 10, wherein the fuel cell comprises a structure in which said fuelgas and said oxidant gas are supplied from the central parts of saidseparators, respectively, to said fuel electrode layer and said oxidantelectrode layer, respectively, through said fuel electrode currentcollector and said oxidant electrode current collector, respectively.21. The separator for use in a solid oxide fuel cell according to claim13, wherein said surface shape of the separator is formed at least onthe surface in contact with one of the fuel electrode currentcollectors.
 22. The separator for use in a solid oxide fuel cellaccording to claim 14, wherein said surface shape of the separator isformed at least on the surface in contact with one of the fuel electrodecurrent collectors.